An interdental cleaning device in actual use includes a base portion made of a synthetic resin and a soft portion made of an elastomer, in which the base portion includes a handle portion and a core portion provided at the front end of the handle portion and having a slender shaft shape, the soft portion includes a cleaning soft portion covering the core portion, the handle portion forms a gripping portion as a grip, and the core portion and the cleaning soft portion form a cleaning portion for cleaning between teeth (see, e.g., Patent Literatures 1 to 3).
A method widely used to manufacture this interdental cleaning device includes charging a synthetic resin material into a first molding space of first molds to form a base portion; placing, in a second molding space of second molds, the base portion molded in the first molds; and charging an elastomeric material into the second molding space to form a soft portion, so that the interdental cleaning device is obtained. In general, a plurality of interdental cleaning devices are also molded simultaneously by a process that includes providing first molds having a plurality of first molding spaces and second molds having the same number of second molding spaces as the first molding spaces; supplying a synthetic resin material to the first molding spaces from a common runner to simultaneously form a plurality of base portions; placing, in the second molding spaces of the second molds, a primary molded product having the base portions connected with a runner; and then charging an elastomeric material into the second molding spaces through a common runner, so that a plurality of interdental cleaning devices are simultaneously formed.
In this regard, the mold structure should be made as simple as possible. Therefore, the elastomeric material is generally charged into each second molding space from the front end side to base end side of the core portion of the base portion, loaded in the second molding space, through a gate placed on the front end side of the interdental cleaning device in the second molding space. In addition, the elastomeric material has relatively high viscosity. Therefore, if the elastomeric material is charged from the base end side to front end side of the core portion during the molding of the cleaning soft portion in the second molding space, poor charging may occur at the front end of the cleaning soft portion. Also from this point, therefore, the elastomeric material is generally charged from the front end side to base end side of the core portion so that poor molding can be prevented at the front end of the cleaning soft portion.
As mentioned above, the method of charging the elastomeric material from the front end side to base end side of the core portion can make the mold structure simple and prevent poor molding at the front end of the cleaning soft portion. However, this method raises problems as described below. When this method is used, the opening area of the gate portion cannot be made large, so that the amount of injection of the elastomer cannot be increased, which can make it impossible to charge a sufficient amount of the elastomeric material into the gripping portion side, make it difficult to form a non-slip portion or other portions on the gripping portion, and lead to a quick decrease in the temperature of the charged elastomer. In this method, therefore, it can be difficult to bond the elastomeric material to the synthetic resin of the core portion, so that the non-slip portion may easily peel off. In order to prevent the front end of the core portion from being softened and deformed, this method also forces the core portion to have a large diameter, which can make it impossible to insert the cleaning portion between teeth or can reduce the length of the cleaning portion insertable between teeth, so that sufficient cleaning ability cannot be obtained.
In this regard, Patent Literature 4 proposes an interdental cleaning device-manufacturing method that includes retaining the elastomeric material in an annular reservoir space formed along the circumference of the handle portion in the second molding space; and supplying the elastomeric material from the reservoir space uniformly to the entire circumference of the base portion of the cleaning portion molding space through a plurality of connecting portion molding spaces formed along the handle portion.